Best way to color only the faces that are not debossed
If you want something quick and dirty, you could create a mask based on the axis co-ordinates of the faces of your &quot;lettering&quot; (I used X axis in my example). An unfortunate side effect, however, is that it leaves a black stripe around the edge of the object at the same X-position:

If instead, you want to select the letter faces and assign them a different material (ie: you just need a quick way to select all the faces at a certain &quot;depth&quot;), select one of the faces at that depth and then select &gt; similar &gt; co-planar. You can then assign a different material to the selected faces: